net: ethtool: add support for ETHTOOL_GRXRINGS ioctl

net: ethtool: add support for ETHTOOL_GRXRINGS ioctl

This patch adds handling for the ETHTOOL_GRXRINGS ioctl command in the
ethtool ioctl dispatcher. It introduces a new helper function
ethtool_get_rxrings() that calls the driver's get_rxnfc() callback with
appropriate parameters to retrieve the number of RX rings supported
by the device.

By explicitly handling ETHTOOL_GRXRINGS, userspace queries through
ethtool can now obtain RX ring information in a structured manner.

In this patch, ethtool_get_rxrings() is a simply copy of
ethtool_get_rxnfc().

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250917-gxrings-v4-2-dae520e2e1cb@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>